Field responsive photoluminescent display devices

ABSTRACT

Improved image storage and display devices of the type which includes a layer of phosphor material which is stimulated to emit light through photoluminescence. The intensity of emitted light is varied by applying an electric field to the phosphor layer. The field may be applied to the phosphor layer by sandwiching the layer between a layer of insulating material and a layer of electrically conductive material and subjecting the insulator to an electron beam. To alter the field strength at selected locations a writing electron beam is focussed on the insulator at these selected locations. In the alternative, the phosphor layer may be covered on one surface with insulation materials forming a charge trap and the phosphor and insulation layers sandwiched between sets of orthognal electrically conductive strips. Selective field alteration is accomplished by X-Y addressing an orthognal pair of conductive strips.

United States Patent 1191 Kazan et al.

[ FIELD RESPONSIVE II-IO'IOLUMINESCENT DISPLAY DEVICES [75] Inventors:Benjamin Kazan; Thomas O.

Sedgwick, both of Westchester, NY.

1451 Apr. 1, 1975 [73] Assignee: International Business MachinesCorporation, Armonk, NY. [57] ABSTRACT [22] Filed: Dec. 26, 1972 211App]. No: 318,136

improved image storage and display devices of the type which includes alayer of phosphor material which is stimulated to emit light throughphotolumi 52 5 (1 H 315 11 315 12 3 3 5 AB nescence. The intensity ofemitted light is varied by 313/92 250 3 5 applying an electric field tothe phosphor layer. The 511 Int. Cl. 1101, 31/48 field may be applied tothe P p layer y sand- [58] Field of Search .1 315/11, 12; 313/65 AB,wishing the layer between a layer of insulating mate- 313/68 A, 92 PH;250/365 rial and a layer of electrically conductive material andsubjecting the insulator to an electron beam. To alter 5 ReferencesCited the field strength at selected locations at writing elec- UNITEDSTATES PATENTS tron beam is focussed on the insulator at these se 7 S Iq lected locations. in the alternative, the phosphor layer z g l g2 maybe covered on one surface with insulation materi- 4/196; Turrllcer e d Xals forming a charge trap and the phosphor and insula- 3'437'752 4mm;Ford g 5 X tion layers sandwiched between sets of orthognal elec-3:594:607 7/l97l Frankland 313/68 A x trically conductive stripsSelective field alteration is 3.051.362 3/1972 Takita 313/68 A Xaccomplished by X-Y addressing an orthognal pair of 3,663v8l3 5/1972Shaw 250/365 X conductive strips. 3.666346 5/l972 Trimhlc ,1 250/365 XFOREIGN PATENTS OR APPLICATIONS 8 Claims, 3 Drawing Figures 2.05l.0993/l972 France VIEWING DIRECTION 3 X A l- 1 7w Li 4 30 x m m i I iii m 4I Z O 0 Y (Zn 0) 1 2 1 4 U V, TRANSMITTING GLASS SUBSTRATE *JENTEQHFB 1i975 4.875.457

14 B L, HIGH VOLTAGE FIG j T SUPPLY 12 EXCITATION I f,v| |BL Q u HTINSULATING FILM (Si0 03 H PHOSPHOR LAYER (zn0)- 4 2 METAL BACK PLATEWRITING sun 26 l 2kV VIEWING DIREETION i 23 L X 3 f///////////////////////////7 X L FL/////////////////////////// Z L Q g(5i 0 2 2 f f 32 [Zn 0) 1 2 3 4 U. V. TRANSMITTING H r I I GLASSSUBSTRATE 3 /s d HcE 14 FIELD RESPONSIVE PHOTOLUMINESCENT DISPLAYDEVICES BACKGROUND OF THE INVENTION 1. Field of the Invention Theinvention is in the field of image storage and display devices andparticularly pertains to those devices which use phosphor materialexcited to emit light through the phenomenon of photoluminescence.

2. Description of the Prior Art Various image storage and displaydevices using phosphor materials are known. In one type of device, thereis provided a cathode ray tube, including a target comprised of a layerof phosphor material excited to emit light by flooding the target withan electron beam produced by a flooding electron gun. To control theintensity of luminescence at selected areas of the phosphor material,the cathode ray tube includes a storage grid upon which is stored acharge image corresponding to the light image to be emitted by thetarget. This stored charge image controls the electron flow between theflooding electron gun and the target to thereby modulate the intensityof the target luminescence in accordance with the stored charge image.Such storage and display devices are explained in greater detail in US.Pat. No. 3,002,124 to Schneeberger, issued Sept. 26, 1961, US. Pat. No.3,567,984 to Allard, issued Mar. 2, 1971 and US. Pat. No. 3,243,644 toRoe, issued Mar. 29, 1966.

These image storage and display devices suffer the disadvantage thatcontinuous luminescence of the target requires a continuous flooding ofthe phosphor layer with electrons and thus the continued energization ofthe flood beam. If the flood beam is cut off, the light output dropsinstantly. Not only does this cause an excessive use of electricalenergy, but requires a relatively complicated structure.

It is also known that phosphor materials can be made to emit lightthrough photoluminescence and that the intensity of the emitted lightcan be controlled by a direct application of voltage to the material.For example, it is known that phosphor materials such as ZnO and ZnS canbe made to emit light when subjected to ultraviolet radiation and thatthe intensity of this emitted light can be controlled by directlyapplying an electric potential to the phosphor material. For adiscussion of this phenomenon the reader is directed to the article byP. F. Daniels et al, Control of Luminescence by Charge Extraction,"Physical Review, Vol. 111, Sept. 1, 1958, pages 1240-1244 and thearticle by L. W. Hershinger et al, Very Low Voltage DCElectroquenchable," Abstract of Electrochemical Society Meeting, May3-7, 1959.

Although these articles suggest a technique for varying the intensity ofluminescence in phosphor materials excited by photoluminescence, thereis no suggestion of providing an image display device exhibiting highresolution nor a device which has the capability of both image storageand display. With respect to image storage, the technique described inthe aforementioned articles requires a sustained applied potential withassociated current flow, in order to retain the intensity of the emittedlight at one of two levels. As soon as the applied potential and currentis removed, the intensity of luminescence changes.

SUMMARY OF THE INVENTION The present invention is directed to animproved image storage and display device using phosphor materialsexcited to emit light by photoluminescence. In accordance with theteaching of the invention, the display device is provided with an imagestorage and display feature that does not require the direct applicationof a potential to the phosphor material. We have discovered that theintensity of the light emitted by the phosphor material can be changedby subjecting the material to an electric field without the directapplication of a potential to the material. The intensity remains atthis new level so long as the field exists.

In a basic embodiment of the invention, a phosphor material, such as ZnOor ZnS, is coated with an insulator on one surface thereof and aconductive backplate on the opposite surface to produce an electricfield when the insulator is bombarded with electrons. When the phosphorlayer is subjected to radiation, it emits lights. For a ZnO or ZnSphosphor layer, luminescence results in response to ultravioletexcitation. The intensity of luminescence is less when the phosphorlayer is subjected to an electric field. The luminescence remains at alower intensity level as long as the field remains. While the field isinitiated by bombarding the insulator with an electron beam, the fieldis retained over a relatively long period of time in the absence of theelectron beam by virtue of the charge storage capability of theinsulating material. Thus, the phosphor materials continue to luminesceat the lower intensity level after the electron beam has beenextinguished.

In a second embodiment of the invention, the basic embodiment ismodified to provide a new type of display storage tube of the typeincluding a target comprised of a layer of phosphor material. The tubeincludes a writing gun producing an information modulated writingelectron beam which alters the electric field at selected points on thephosphor material in accordance with the writing beam modulation.

In the third embodiment, the basic embodiment is modified to provide anX-Y addressed storage and display panel.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the basicembodiment of the invention;

FIG. 2 illustrates a modification of the embodiment of FIG. 1 to providea high resolution image storage and display tube; and

FIG. 3 illustrates a third embodiment of the invention which includesthe X-Y addressing of selected locations on the phosphor material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates afirst embodiment of the present invention. This embodiment will be usedto explain the basic operation of the inventive device. The device ofFIG. I is comprised of a display panel including a phosphor layer 2, forexample a ZnO or ZnS layer. One surface of the phosphor layer 2 iscovered with an insulating film 4, which, by way of example only, maybeSiO,. The opposite surface of the phosphor layer is covered by ametallic backplate 6, illustrated as being opaque. This conductivebackplate 6 may, however, be transparent or translucent. The phosphormaterial 2 is caused to luminesce through the phenomenon of photoluminescence. When the phosphor material 2 is ZnO or ZnS, luminescenceresults when the material is subjected to ultraviolet radiation fromsource 14. For ease in understanding the present invention, theinvention wiil be explained assuming the phosphor layer to be ZnO andthe excitation ultraviolet radiation. It is to be understood, however,that the present invention is not limited to Zn) phosphor material norto ultraviolet ex citation.

The level of luminescence is controlled by flooding the insulating layer4 with a defocused electron beam produced by a suitable electron gunshown, by way of example, in FIG. I, as a corona generator 8, thecathode of which is connected to the negative terminal ofa high voltagepower source 12. By negatively charging the insulator, using the coronagenerator in air, it has been found that the brightness of the ZnO layer2 can be decreased to less than one-fourth its initial value. The levelof luminescence then begins to increase over a period of several minutesto a new equilibrium level which is about half the initial brightness.Although the reason for this change in brightness level is notcompletely understood, it is believed that the reduction in light outputis caused by upward bending of the bands of the ZnO at the ZnO-SiOinterface which interferes with the photoluminescence process.

By making use of the above field effect control of photoluminescence, anew type of display storage tube can be built as illustrated in FIG. 2.This tube comprises a conventionally shaped tube including a transparentor translucent glass faceplate 22 at the front thereof. Directly behindthe faceplate 22 is placed a target or screen 7 consisting of atransparent conductive layer [8, phosphor layer 2 and insulating layer4. Common elements in FIGS. 1, 2 and 3 will be designated by commonnumerical designators. Situated behind target 7 is a collector mesh 15.Photoexcitation of the phosphor layer is produced by the radiationsource 14, which is shown, by way of example in FIG. 2, as anultraviolet source. The tube wall, the collector mesh 15, as well as theinsulating layer 4, are made from materials transparent to ultravioletradiation, so that the phosphor layer 2 may be excited intoluminescence. The tube also includes an electron flooding gun 28 and awriting electron gun 26, the function and operation of which will bedescribed in detail hereinbelow.

The operation of the display tube will now be described. To place thetube in its initial or erased state, the flooding gun 28 is turned onwhile the potential of conductive layer 18 is pulsed to IOO with respectto ground by closing switch 17 for a short period of time therebyconnecting the conductive layer to source 19. With switch 17 closed,capacitor 23 charges to l00 volts. When switch I7 is opened, capacitor17 begins to discharge through resistor 25. The values of the capacitor23 and resistor 15 are selected so that the dis charge time of thecapacitor is several tenths of a second.

The energy of flooding beam 29 is controlled, by setting the cathodepotential of electron gun 28 in such a way that during a substantialportion of the discharge interval the ratio of the number of secondaryelectrons emitted by insulating layer 4 to the number of primaryelectrons of beam 29 is less than unity. In the embodiment illustratedinsulating layer 4 is SiO and the cath- Ode of gun 28 is set at -l 00V.In this erased condition,

excitation of ZnO layer 2 by ultraviolet source 14 re sults in a lowlevel of light output.

To accomplish a writing operation, flooding beam 29 is turned off andinsulating layer 4 is scanned by a writing beam 27 produced by writingelectron gun 26. The position of writing beam 27 is controlled byelectrical input signals in a conventional manner; for example, theposition of the writing beam can be controlled in accordance with asuitable input signal to deflecting plates 24. In the preferredembodiment, the cathode of writing gun 26 is set at 2k\/.

The setting of the potential of the cathode of the writing gun is suchthat the energy of writing beam 27 contains sufficient energy to renderthe ratio of the number of secondary electrons freed from the insulatinglayer to the number of primary electrons of the writing beam greaterthan unity. The result is that where insulating layer 4 is struck by thefocused writing beam the potential rises close to ground potential asgrounded collector mesh 15 substantially absorbs all electrons emittedsecondarily. The reduction of the electric field over the phosphor layerat areas of the insulating layer where the writing beam has struck, willresult in an increase in the efficiency of luminescence at correspondingareas of phosphor layer 2 producing a bright trace on a relatively darkbackground. Assuming no charge leakage through insulating layer 4, thestored image may be viewed for many minutes after writing beam 27 is cutoff.

To maintain an image for very long periods of time, the flooding beam 29is switched on while the conductive plate 18 is connected to groundpotential. The action of the flooding beam will be to maintain the areason the insulating layer 4 at either one of two stable potential states;lOO volts, corresponding to the erase state, at those areas not scannedby beam 27, and substantially ground at those locations which have beenscanned by the writing beam 27.

More specifically, those areas of the target which have not been writtenupon, may tend to leak charge through phosphor layer 2, and thetransparent conductive layer 18 to ground. Thus, the potential at thepoints where charge leakage has occurred may rise to some potentialabove l0() volts and the luminescence at these points begin to increase.As the flood beam strikes these areas, the charge thereon returns to -l00 volts, assuming the potential at those points has not risensufficiently due to leakage such that the energy of the flooding beam 29would be sufficient to cause secondary emission with a factor greaterthan unity. How ever, at the locations which have been scanned by thewriting beam 17, the potential is sufficiently more positive than thatof the flooding beam 29 such that the energy of the beam with respect tothose areas is sufficient to cause secondary emission with a factorgreater than unity. Therefore, the number of secondary electrons atthese locations is greater than the number of primary electrons causingthese locations to continue to luminesce brightly.

As indicated above, to erase an image, switch 17 is closed momentarilycausing the potential of the transparent conductive layer 18 to drop toJ00 volts with respect to ground and slowly rise to ground potentialwhile the flooding beam 29 is on. As a result, the energy of the floodbeam is insufficient, during a substantial portion of the erase time, tocause secondary emission with a factor greater than unity, and theentire insulating layer assumes a potential l() volts.

The above-described image storage and display tube 20 can be modified bydeleting the flooding gun 28, in which case the writing gun 26 is usedfor flooding purposes by defocusing the writing beam while its cathodeis set at 100 volts rather than 2 kilovolts.

Because of the continuous nature of the phosphor layer 2 and insulatinglayer 4, high resolution stored images can be produced. As an example ofthe brightness obtainable, a 5 watt black light source can excite aphosphor area of about 5 inches by 5 inches to foot- Lamberts brightnessor higher. Since the life of the ZnO layer under ultraviolet excitationis very long, the reliability of the tube is extremely great.

A further embodiment of the invention is illustrated in FIG. 3. In thisembodiment, addressing of the display screen is provided by a pluralityof X-Y conductors rather than through the use of electron beams. Morespecifically, the display screen is comprised of a layer 2 of phosphormaterial, one surface of which is covered by an insulating layer 4 of afirst insulating material such as SiO Above this first insulating layer4 is positioned a second insulating layer 30 of a second insulatingmaterial such as Si N the relative thicknesses of the two insulatinglayers being controlled to provide charge trapping in the manner wellknown in the field of silicon memory technology to permit sustainedexcitation. Above the Si N insulating layer 30 is positioned a pluralityof parallel, semi-transparent conductive strips X,X On the surface ofthe phosphor layer 2 opposite the surface bearing the SiO layer 4 ispositioned a plurality of parallel semi-transparent conductive stripsY,-Y situated orthogonal to the conductive strips X -X The displayscreen is preferably supported on a glass substrate 32 which istransparent to the phosphor exciting light generated by source 14. Feweror additional X and Y conductors can be used as desired.

In this configuration, a negative charge storage at the SiO Si l linterface will produce a depleted surface on the ZnO, thus loweringluminescence when excited by source 14. By comparison, a positive orzero charge stored at the SiO Si;,N,, interface will result inrelatively high luminescence. To alter the charge at the insulatingmaterial interface at a local region, coincident pulses of W2 and V/2are applied to a pair of selected X-Y conductors. Thus, only the regionbetween the selected conductors will experience the full pulse of avoltage sufficient to exceed some threshold voltage V where V/2 V V.This results in a tunnelling of the carriers from the ZnO through theSiO to the Si O -SEN, interface. This charge may be removed by applyingvoltage pulses of opposite sign to the selected conductors.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. An image storage and display device comprising: a solid layer ofphosphor material having formed on one surface thereofa layer ofelectrically insulating material and on its opposite surface a layer ofconductive material; means for exposing said phosphor material to lightto stimulate it to a level of luminescence dependent upon the electricfield across said phosphor layer. and

means for applying a charge pattern to said insulating layer resultingin an electric field pattern across said phosphor layer corresponding tosaid charge pattern.

2. The device of claim 1 wherein said means for exposing said phosphormaterial to light includes means for producing light in the ultravioletregion of the spectrum.

3. The device of claim 1 wherein said means for applying a chargepattern comprises means for locally charging the insulating layer withelectrons.

4. The device of claim 3 wherein said means for applying a chargepattern further includes writing means for producing a focussed writingbeam of electrons, said writing means including means for controllingthe position of said writing beam.

5. The device of claim 1 wherein said means for applying a chargepattern comprises an electronic gun means for producing an electron beamimpinging said insulating layer, said electron gun means including meansfor selectively focussing and defocussing said electron beam and meansfor altering the energy of said electron beam.

6. The device of claim 1 further including a second layer ofelectrically insulating material disposed over said layer of insulatingmaterial, said means for applying a charge pattern comprising a firstset of conductive strips disposed over said second insulating layer anda second set of conductive strips, orthogonal to said first set formingsaid layer of conductive material.

7. In an image display device including a layer of phosphor materialstimulated to luminesce by photoluminescence, the improvementcomprising:

a continuous layer of insulating material formed over one surface ofsaid phosphor layer and a continuous layer of conductive material formedover the opposite surface of said phosphor layer, light sourcestimulating said phosphor layer to a level of luminescence dependentupon the electric field across said phosphor layer,

a writing electron gun for subjecting selected locations on saidinsulating layer with a focussed electron beam to thereby change theintensity of the electric field acting on said phosphor layer at saidselected locations, and a flooding electron gun means for flooding theinsulating layer with electrons to sustain the local electrical fieldsacting on the phosphor layer. 8. The device of claim 7 wherein saidphosphor material is ZnO, said insulating material SiO and said radiation source a source of ultraviolet radiation.

1. AN IMAGE STORAGE AND DISPLAY DEVICE COMPRISING: A SOLID LAYER OFPHOSPHOR MATERIAL HAVING FORMED ON ONE SURFACE THEREOF A LAYER OFELECTRICALLY INSULATING MATERIAL AND ON ITS OPPOSITE SURFACE A LAYER OFCONDUCTIVE MATERIAL; MEANS FOR EXPOSING SAID PHOSPHOR MATERIAL TO LIGHTTO STIMULATE IT TO A LEVEL OF LUMINESCENCE DEPENDENT UPON THE ELECTRICFIELD ACROSS SAID PHOSPHOR LAYER. AND MEANS FOR APPLYING A CHARGEPATTERN TO SAID INSULATING LAYER RESULTING IN AN ELECTRIC FIELD PATTERNACROSS SAID PHOSPHOR LAYER CORRESPONDING TO SAID CHARGE PATTERN.
 2. Thedevice of claim 1 wherein said means for exposing said phosphor materialto light includes means for producing light in the ultraviolet region ofthe spectrum.
 3. The device of claim 1 wherein said means for applying acharge pattern comprises means for locally charging the insulating layerwith electrons.
 4. The device of claim 3 wherein said means for applyinga charge pattern further includes writing means for producing a focussedwriting beam of electrons, saiD writing means including means forcontrolling the position of said writing beam.
 5. The device of claim 1wherein said means for applying a charge pattern comprises an electronicgun means for producing an electron beam impinging said insulatinglayer, said electron gun means including means for selectively focussingand defocussing said electron beam and means for altering the energy ofsaid electron beam.
 6. The device of claim 1 further including a secondlayer of electrically insulating material disposed over said layer ofinsulating material, said means for applying a charge pattern comprisinga first set of conductive strips disposed over said second insulatinglayer and a second set of conductive strips, orthogonal to said firstset forming said layer of conductive material.
 7. In an image displaydevice including a layer of phosphor material stimulated to luminesce byphotoluminescence, the improvement comprising: a continuous layer ofinsulating material formed over one surface of said phosphor layer and acontinuous layer of conductive material formed over the opposite surfaceof said phosphor layer, a light source stimulating said phosphor layerto a level of luminescence dependent upon the electric field across saidphosphor layer, a writing electron gun for subjecting selected locationson said insulating layer with a focussed electron beam to thereby changethe intensity of the electric field acting on said phosphor layer atsaid selected locations, and a flooding electron gun means for floodingthe insulating layer with electrons to sustain the local electricalfields acting on the phosphor layer.
 8. The device of claim 7 whereinsaid phosphor material is ZnO, said insulating material SiO2, and saidradiation source a source of ultraviolet radiation.